\bibitem{Schmidt04}     A.~Schmidt, S.~Zidowitz, A.~Kriete, T.~Denhard, S.~Krass, H.-O.~Peitgen,
                        {\it A Digital Reference Model of the Human Bronchial Tree},
                        Comp.~Med.~Imag.~Graph. {\bf 28}, 203, 2004.